Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers, and the like, are configured to communicate over wireless networks. Accordingly, each such portable communication device relies on a receiver and a transmitter, typically connected to a single or common antenna, for sending and receiving data and control signals over the wireless network. Of course, the receiver and transmitter may be incorporated into a transceiver, having a receiver portion and a transmitter portion. In order to use the common antenna, a duplexer is included to interface between the antenna and each of the receiver portion and the transmitter portion, so that the receiver portion is able to receive signals on a receive (downlink) frequency, and the transmitter portion is able to send signals on a different transmit (uplink) frequency. Generally, the duplexer includes two band-pass filters having different passbands for filtering the receive and transmit signals, respectively, thus preventing or reducing interference between the receive and transmit signals.
Various types of wireless network are implemented according to different communication standards, such as universal mobile telecommunications system (UMTS), global system for mobile communication (GSM), personal communications services (PCS), digital cellular system (DCS), international mobile telecommunication (IMT), and enhanced data rates for GSM evolution (EDGE). The communication standards identify separate bands for transmitting and receiving signals. For example, UMTS band 2 (PCS) provides an uplink frequency band of 1850 MHz-1910 MHz and a downlink frequency band of 1930 MHz-1990 MHz; UMTS band 3 (DCS) provides an uplink frequency band of 1710 MHz-1785 MHz and a downlink frequency band of 1805 MHz-1880 MHz; UMTS band 7 (IMT-E) provides an uplink frequency band of 2500 MHz-2570 MHz and a downlink frequency band of 2620 MHz-2690 MHz; and UMTS band 8 (GMS-900) provides an uplink frequency band of 880 MHz-915 MHz and a downlink frequency band of 925 MHz-960 MHz. Accordingly, a duplexer operating in compliance with a UMTS standard would include a transmit filter having a passband within the corresponding uplink frequency band, and a receive filter having a passband within the corresponding downlink frequency band.
As mentioned above, a duplexer includes two band-pass filters having different passbands for filtering the receive and transmit signals, respectively. The band-pass filters may include acoustic resonators, such as surface acoustic wave (SAW) resonators or thin film bulk acoustic resonators (FBARs), for example, for filtering the receive and transmit signals. Impedance matching circuits are needed to enable the duplexer to interface with the receive and transmit portions of a transceiver, respectively.
One difference between a band-pass filter having SAW resonators (SAW filter) and a band-pass filter having FBARs (FBAR filter) is that the SAW filter has a differential input/output for differential (balanced) signals, while the FBAR filter has a single-ended input/output for single-ended (unbalanced) signals. Therefore, a SAW filter is able to interface directly with the receive or transmit portion of the transceiver, which likewise have a differential input/output, while the FBAR filter relies on balun circuits (in addition to the impedance matching circuits), for example, to convert between single-ended and differential signals in order to interface with the transmit and receive portions of the transceiver. Therefore, the circuitry and layout architecture for interfacing the different types of duplexers with transceivers are not interchangeable. For example, a portable communication device printed circuit board (PCB), designed to include a transceiver and a duplexer having SAW filters, does not include circuitry or space for balun circuits. Thus, a different PCB design would be required to accommodate a duplexer having FBAR filters.